Heat, freezing, dehydration, and high osmotic pressure present serious stresses to a large number of plants and animals in the biosphere. In the study of the biology of adaptation it is found that organisms adapted to extreme environments accumulate significant intracellular concentrations of solutes termed 'osmolytes'. Osmolytes protect proteins and other cell components against the extreme conditions and do so without significantly altering the biological activity of the macromolecule. These characteristics functionally describe these osmolytes as 'compatible'. In adaptations in which urea is concentrated in cells, as in sharks and in mammalian kidney, methylamine osmolytes are concentrated intracellularly to protect cell proteins against the deleterious effects of urea. These osmolytes are said to be 'counteracting' and are believed to alter the biological activity of proteins in a direction opposite to that of urea. The long term goals of the proposed research are two fold: (I) to understand the mechanisms by which natural occurring osmolytes protect proteins against thermal inactivation, inactivation by denaturing solvents, and inactivation brought about by drying, and (II) to explore how osmolytes greatly affect protein stability without causing much of an effect on biological activity and protein structure. To understand the origin of the protecting effects of osmolytes we have begun determining transfer free energies of amino acids (deltaGtr) from water to osmolyte solutions. We have found that the solvophobic character of amino acids are qualitatively different in osmolyte solutions than they are in water, with the aromatic side chains favoring transfer to osmolyte and the aliphatic side chains opposing it. All side chain deltaGtr values are, however, small in magnitude and appear to contribute little to stabilization of the native state of the protein in osmolyte solutions. The most striking result of these studies so far is that the unfavorable transfer of the polypeptide backbone from water to osmolyte is the dominant factor preventing unfolding and therefore contributing to protein stability. It is clear that different principles appear to be operating when protein unfolding is being carried out in the presence of osmolytes as compared to water. The specific aims of this work is to investigate the-fundamental properties of side chains and peptide backbones in these solvent systems, to determine the effects of osmolyte concentration on enzyme kinetic parameters, and to evaluate the flexibility and internal dynamics of proteins in osmolytes by amide exchange kinetics using 2D NMR.